Low-noise type traveling wave tube with an electrostatic field retarding spurious electron flow



June 13, 1967 c. BIGUENET 3,325,669

LOW-NOISE TYPE TRAVELING WAVE TUBE WITH AN ELECTROSTATIC FIELD RETARDING SPURIOUS ELECTRONFLOW Filed Dec. 1962 INVENTOR Chsleuzucr 1 ATTORNE United States Patent 3,325,669 LOW-l (ESE TYPE TRAVELING WAVE TUBE WITH AN ELEETROSTATIC HELD RETARDING SPU- RIQUS ELECTRQN FLGW Charles Biguenet, Paris, France, assignor to CSF Compagnie Generate de Telegraphic Sans Fii, Paris, France Fiied Dec. 4, 1962, Ser. No. 242,292 Claims priority, application France, Dec. 7, 1961, 381,225 Patent 1,315,169 7 Claims. (Cl. 315-35) The present invention relates to traveling wave tubes of the O-type, which may be either forward or backward amplifiers or oscillators, and aims at suppressing or at least reducing the noise or at least a portion of the noise due to certain causes which degrade the performances of this type of tubes.

In the usual tubes of this type, the beam is delivered by a gun comprising generally two anodes, of which the second anode is carried at the potential of the delay circuit in such a manner as to define an interaction space without electrostatic field. The second anode is additionally realized constructively in such a manner as to shield the gun space with respect to the interaction space. The first anode, located within the gun space in front of the second anode, is carried at a reiatively low positive potential, rather close to that of a cathode and very much lower to that of the delay circuit.

Extensive research by applicant has revealed a phenomenon which may be held responsive for a large portion of the noise which affects the operation of the O-type tubes, it being understood however, that the validity of the claims herein is not dependent on the accuracy of the underlying theory to be described hereinafter. This phenomenon consists in the liberation of secondary electrons as a result of the impact of certain electrons of the beam on the first element of the delay line whereby these secondary electrons displace themselves within the space without field at low speed in counter flow to the primary beam, that is, substantially in the opposite direction, and produce by interaction with the electrons of the primary beam, the starting of parasitic oscillations of which the frequency is of the order of 2 to megacycles. These oscillations, which produce noise, in turn excite plasma oscillations within the beam, which plasma is formed by the mixture of electrons of the beam with the ions of the residual gasses within the tube, ions which are carried by the troughs of the potential formed within the beam by reason of the space charge thereof. These plasma oscillations add still further noise though at frequencies clearly lower, for example, between 400 and 1200 kilocycles.

The present invention provides means for suppressing the aforementioned displacement of the secondary electrons issued from the first element of the delay line in counterfiow to the principal beam which, in turn, suppresses the parasitic oscillations described hereinabove and therefore the plasma oscillations while also eliminating the noise resulting therefrom.

According to the present invention, there is used a gun provided with one single anode at a potential very much lower than that of the delay line, in combination with a particular form of the first element of the delay circuit such that the shielding between the gun space and the interaction space is incomplete but that the beam nevertheless is sufficiently defined, taking into consideration the desired band of electronic tuning of the tube. Because of this incomplete shielding, the electric field of the gun space penetrates slightly into the first portion of the interaction space and creates therein a component which opposes the undesired movement of the secondary electrons. The first element of the delay line according to the 3,325,559 Patented June 13, 1961 ice present invention is therefore larger than the normai elements but will be pierced by one or several window: forming an electron-optical system suitable to assure the desired definition of the beam.

Accordingly, it is an object of the present invention tc provide a traveling wave tube structure, and more particularly an O-type traveling wave tube structure which eliminates in a simple and effective manner the shortcomings encountered in the prior art constructions.

It is another object of the present invention to provide an O-type traveling wave tube, either amplifier or oscillator of the forward or backward type, in which the noise or at least a substantial portion of the noise is substantially eliminated.

A further object of the present invention resides in the provision of an O-type traveling wave tube which, by simple means, effectively reduces in an extremely reliable and effective manner, noises due to parasitic oscillations.

A still further object of the present invention resides in the provision of means associated with an O-type traveling wave tube which effectively prevents the counterflow of secondary electrons emitted from the initial part of the delay line which are susceptible of producing disturbances and noises in the tube during operation thereof.

Still another object of the present invention resides in the provision of means associated with an O-type traveling wave tube which considerably improves the operating characteristics thereof especially as regards the signal to noise ratio.

These and other objects, features and advantages of the present invention will become more obvious from the following description when taken in connection with the accompanying drawing which shows, for purposes of illustration only, one embodiment in accordance with the present invention, and wherein FIGURE 1 is a longitudinal cross sectional view of an O-type traveling wave tube, for example, a backward wave oscillator utilizing the means in accordance with the present invention;

FIGURE 2 is a cross sectional view, taken at right angles to the cross sectional view of FIGURE 1, of a portion of the tube illustrated in FIGURE 1, and

FIGURE 3 is a lateral view of the first finger of a delay line in accordance with the present invention as used with the tube of FIGURES 1 and 2.

Referring now to the drawing wherein like reference numerals are used throughout the various views to designate like parts, the illustrated embodiment represents an O-type backward wave oscillator or Carcinotron provided with an inter-digital delay line along both sides of which flow two parallel beams in the form of ribbons. Consequently, in the illustrated embodiment there has been shown an electron gun, of any known conventional construction, which supplies such a pair of beams. Since such a gun is well known in the art, a detailed description thereof is dispensed with herein. The gun which is located within an evacuated metallic enclosure 1 comprises a cathode 2, a Wehnelt electrode 3 and a single anode 4. The electrode 3 is carried at the negative potential of the source 5 with respect to the cathode 2 whereas the anode 4 is carried at a positive potential with respect to the cathode 2 but very much lower than the positive potential at which is carried the envelope 1 with respect to the cathode 2. The electrodes 3 and 4 are each provided with two windows 6, 7 and 8, 9, respectively, through which pass the laminar beams 10 and 11.

The envelope 1 contains in addition an interdigital line 12 of which the last elements 13 and 14 are covered with an attenuation as usual in backward wave oscillator tubes. This line 12 is fixed to the walls of the envelope 1 and is,

therefore, carried at the same potential as the latter. The

3 envelope 1 terminates in a portion 15 forming the collector for the beams and 11. The tube is placed within 1 longitudinal magnetic focusing field, indicated only by the arrow B, the illustration of the source of the magnetic field being omitted herein to simplify the drawing.

The first finger 16 of the delay line 12 is of a particular shape and form. As shown in FIGURES 1 and 3, the first finger 16 is wider than the normal fingers, and is provided additionally with two windows 17 and 1-8 for the passage therethrough of the beams 10- and 11, the position of these windows 17 and 18 being such that the beams 10 and 11 pass close to the normal fingers of the delay line 12 on both sides thereof. Furthermore, this finger 16 is longer than the normal fingers so as to extend through the insulating passage 19 to the outside of the envelope 1 and to be able to be connected with a line 20 through which the high frequency output energy of the backward wave oscillator is removed.

This finger 16 therefore shields incompletely the interaction space 23 adjacent the delay line with respect to the gun space 24. It leaves in effect the passages 21 and 22 through which may slightly penetrate into the first portion of the interaction space 23 the electrostatic field created between the envelope 1 and the anode 4.

The operation of this tube as backward wave oscillator does not require any explanation since it is well known per se. Nevertheless, the operation thereof is substantially and significantly improved in accordance with the present invention by the suppression or radical reduction of noise which is attributed by the inventor to the effect of penetration of the electrostatic field through the passages 21 and 22 into the first portion of the space 23, this field opposing the movement, in a direction opposite to the beams 10 and 11, of secondary electrons liberated by the impact of these beams on the first elements of the line 12.

The improvements according to the present invention may be advantageously combined with other known means for reducing noise, such as the realization of the delay line in an adequate manner to prevent the liberation of seconday electrons by impact from electrons of the beam, for example, by providing the line with a coating having a small coetficient of secondary emission, or with small inclined facets in order to increase the angle of im- 1 pact so that this impact does not skim along the impact surface which, as is known, favors the secondary emission. These facets may be realized by a simple technological process such as sand-blasting, milling, sparking, electrolitic erosion, etc., as is known per se in the prior art. 7 While I have shown and described one embodiment in accordance with the present invention, it is understood that the same is not limited thereto, but is susceptible of all those changes and modifications known to a person skilled in the art. In particular, the present invention does not depend on the particular configuration of the delay circuit nor on the form or number of beams, and is applicable, not only to backward oscillators, but also to amplifiers and other types of O-type oscillators.

Thus, it is obvious that the present invention is not limited to the details shown and described herein, but is susceptible of numerous changes and modifications without departing from the spirit and scope thereof, and I therefore do not wish to be limited to the detail shown and described herein, but intend to cover all such changes and modifications as are encompassed by the scope of the appended claims.

I claim:

1. In an O-type traveling wave tube having, within an evacuated enclosure, an electron gun space adjacent to an interaction space, an electron gun structure including a cathode and an anode in said electron gun space, a delay circuit including a series of periodically spaced elements in said interaction space, means for carrying said cathode at a predetermined negative potential with respect to said delay circuit, and means for carrying the anode at a potential intermediate said cathode and said delay circuit potentials, the improvement of reducing the noise within said tube essentially consisting of locating the first element of said delay circuit elements at a distance close enough to said anode to produce an electrostatic field, said first element being shaped to form passages from said gun space into said interaction space, and said passages being formed between said first element and the walls of said evacuated enclosure to allow a slight penetration of said electrostatic field into said interaction space in the proximity of said first element.

2. In an O-type traveling wave tube having, within an evacuated enclosure, an electron gun space adjacent to an interaction space, an electron gun structure including a cathode and an anode in said electron gun space, a delay circuit including a series of periodically spaced elements in said interaction space, means for carrying said cathode at a predetermined negative potential with respect to said delay circuit, and means for carrying the anode at a potential intermediate said cathode and said delay circuit potentials, the improvement of reducing the noise within said tube which essentially consists of locating the first element of said delay circuit elements at a distance close enough to said anode to produce an electrostatic field, said first element serving to partially shield said interaction space with respect to said electron gun space, said first element having an enlarged form relative to the other delay circuit elements thereby forming with said evacuated enclosure at least one passage for an intercommunication between said interaction space and said electron gun space to allow a slight penetration of said electrostatic field into said interaction space in the proximity of said first element.

3. In an O-type traveling wave tube having, within an evacuated enclosure, an electron gun space adjacent to an interaction space, an electron gun structure including a cathode and an anode in said electron gun space, a delay circuit including a series of periodically spaced elements in said interaction space, means for carrying said cathode at a predetermined negative potential with respect to said delay circuit, and means for carrying the anode at a potential intermediate said cathode and said delay circuit potentials, the improvement of reducing the noise within said tube which essentially consists of locating the first element of said delay circuit elements at a distance close enough to said anode to produce an electrostatic field, said first element serving to partially shield said interaction space with respect to said electron gun space, said first element having an enlarged form relative to the other delay circuit elements thereby forming with said evacuated enclosure at least one passage for an intercommunication between said interaction space and said electron gun space to allow a slight penetration of said electrostatic field into said interaction space in the proximity of said first element, said first delay circuit element having in its surface at least one slot in alignment with the path of the electron beam emitted by said electron gun to enable passage of said beam into said interaction space.

4. An O-type traveling wave tube comprising enclosure means forming effectively an electron gun space and an interaction space, electron gun means within said electron gun space and including cathode and an anode means, wave guide means including a series of periodically spaced elements in said interaction space, and means for reducing the noise within said tube by so arranging said anode means and said wave guiding means as to enable an electrostatic field established between said anode means and said enclosure means to leak into said interaction space so as to oppose counterflow movement of secondary electrons present therein, said last-named means being effectively constituted by locating the first element of said wave guiding means close enou to said anode means to produce said electrostatic field, said first element being of larger size than the other of said P ically Spacedielements so as to incompletely shield said interaction space with respect to said electron gun space to enable said leakage of said electrostatic field.

5. An O-type traveling wave tube comprising enclosure means forming efiectively an electron gun space and an interaction space, electron gun means within said electron gun space and including cathode and an anode means, wave guiding means including a series of periodically spaced elements in said interaction space, and means for reducing the noise Within said tube by so arranging said anode means and said wave guiding means as to enable an electrostatic field set up, during operation of the tube, between said anode means and said enclosure means to leak into said interaction space so as to oppose counterfloW movement of secondary electrons present therein, said last-named means being etfectively constituted by cating the first element of said wave guiding means at a distance close enough to said anode means to produce said electrostatic field, and said first element having an enlarged form relative to the other delay circuit elements thereby forming With said enclosure means at least one passage constituting a leakage path for said electrostatic field between said interaction space and said electron gun space for incompletely shielding said interaction space with respect to said electron gun space.

6. In an O-type traveling wave tube provided with enclosure wall means etfectively forming an electron gun space and an interaction space, electron gun means within said electron gun space and including cathode means and anode means, wave guiding means in said interaction space, means for carrying said cathode at a predetermined negative potential with respect to said wave guiding means, the improvement essentially consisting of means positioned within said interaction space closely adjacent said anode means for producing an electrostatic field within the region of the portion of said wave guiding means near said electron gun space, said means providing passages from said gun space into said interaction spaci along said enclosure wall means to partially shield sai interaction space to provide a slight leakage of said elec trostatic field into said interaction space to oppose coun ter-fiow of secondary electrons emitted within said inter action space.

7. In an O-type traveling wave tube provided with en closure wall means effectively forming an electron gui space and an interaction space, electron means withii said electron gun space and including cathode means 8.I1( anode means, Wave guiding means including a series 0 periodically spacyed elements in said interaction space means for carrying said cathode at a predetermined neg ative potential with respect to said wave guiding means the improvement essentially consisting of said anodi means comprising a single anode structure in said electroi gun space, said first element of said wave guiding mean being positioned closely adjacent said anode means ti produce an electrostatic field, said first element formin; passages between said electron gun space and said inter action space along said enclosure wall means to incom pletely shield said interaction space from said electrostatil field and produce a slight electrostatic field within thi region of said first element in said interaction space, sai electrostatic field having a polarity to oppose the counter flow of secondary electrons emitted within said inter action space.

References Cited UNITED STATES PATENTS 2,936,393 5/1960 Currie et al. 315-1 2,941,113 6/1960 Johnson et al. 315-3.! 3,171,054 2/1965 Dong et al. 3153..' 3,213,314 10/1965 Reverdin 3153..

HERMAN KARL SAALBACH, Primary Examiner.

E. LIEBERMAN, -R. D. COHN, Assistant Examiners. 

1. IN AN O-TYPE TRAVELING WAVE TUBE HAVING, WITHIN AN EVACUATED ENCLOSURE, AN ELECTRON GUN ADJACENT TO AN INTERACTION SPACE, AN ELECTRON GUN STRUCTURE INCLUDING A CATHODE AND AN ANODE IN SAID ELECTRON GUN SPACE, A DELAY CIRCUIT INCLUDING A SERIES OF PERIODICALLY SPACED ELEMENTS IN SAID INTERACTION SPACE, MEANS FOR CARRYING SAID CATHODE AT A PREDETERMINED NEGATIVE POTENTIAL WITH RESPECT TO SAID DELAY CIRCUIT, AND MEANS FOR CARRYING THE ANODE AT A POTENTIAL INTERMEDIATE SAID CATHODE AND SAID DELAY CIRCUIT POTENTIALS, THE IMPROVEMENT OF REDUCING THE NOISE WITHIN SAID TUBE ESSENTIALLY CONSISTING OF LOCATING THE FIRST ELEMENT OF SAID DELAY CIRCUIT ELEMENTS AT A DISTANCE CLOSE ENOUGH TO SAID ANODE TO PRODUCE AN ELECTROSTATIC FIELD, SAID FIRST ELEMENT BEING SHAPED TO FORM PASSAGES FROM SAID GUN SPACE INTO SAID INTERACTION SPACE, AND SAID PASSAGES BEING FORMED BETWEEN SAID FIRST ELEMENT AND THE WALLS OF SAID EVACUATED ENCLOSURE TO ALLOW A SLIGHT PENETRATION OF SAID ELECTROSTATIC FIELD INTO SAID INTERACTION SPACE IN THE PROXIMITY OF SAID FIRST ELEMENT. 